A numerically controlled lathe (hereinafter written as a NC lathe) has been known which machines a work gripped by a chuck of a rotatable spindle while switching tools installed on a plurality of tool rests.
One such NC lathe has a first tool rest moving back and forth in an axis line direction traversing the axis line of the spindle, and a second tool rest provided opposite to the spindle and moving back and forth on the axis line of the spindle, wherein the outer peripheral surface of the work is mainly machined with tools installed on the first tool rest, while the end face of the work is mainly machined with tools installed on the second tool rest.
FIG. 8 shows one example of the NC lathe described above, wherein the configuration of main parts of the NC lathe is shown in a schematic view.
A NC lathe 1 has a head stock 11 rotatably supporting a spindle 12 and capable of moving back and forth in a Z direction which is in the same direction as an axis line L of the-spindle 12, a comb-teeth-shaped first tool rest 13 disposed on one side of the axis line L of the spindle 12, and a comb-teeth-shaped second tool rest 15 disposed opposite to the spindle 12 on the axis line L.
The first tool rest 13 is movable in a Y direction perpendicular to the surface of the drawing of FIG. 8, and in an X direction perpendicular to both the Z and Y directions. Moreover, the second tool rest 15 can move back and forth in the Z direction.
A plurality of tools T1 such as cutting tools is arranged in the Y direction and installed on a tool installation portion of the first tool rest 13. Moreover, the second tool rest 15 is provided with a tool installation portion 16 movable in the X direction on which a plurality of (three in the example shown in the drawing). tools T2 such as drills or end mills for, for example, drilling in an end face of a work W is arranged and installed. Further, the tools T1 mainly machine the outer peripheral surface of the work W, while the tools T2 mainly machine the end face of the work W.
A through-hole is formed in the spindle 12 on the axis line L, and the long rod-like work W is inserted through this through-hole. The work W is gripped by an unshown chuck at the tip of the spindle so that a predetermined length of the tip of the work W protrudes from the front end of the spindle 12. The first tool rest 13 moves in the Y direction to index the predetermined tool T1 to a machining position. Then, by a combination of the movement of the first tool rest 13 in the X direction and the movement of a head stock 31 in the Z direction, the tool T1 is positioned relative to the work W and the work W is cut by the tool T1.
The second tool rest 15. indexes the predetermined tool T2 to a machining position (on the spindle axis line L) by the movement of the tool installation portion 16 in the X direction, and the second tool rest 15 moves in the Z direction to position the tools T2 relative to the work W, thereby machining the end face of the work W with the tools T2.
In the NC lathe 1 having the configuration described above, the second tool rest 15 is on standby at a standby position D sufficiently away from the area for machining the work W during the machining of the work W with the tool T1 installed on the first tool rest 13, as shown. in FIG. 9(a). When the end face of the work W is machined with the tool T2 installed on the second tool rest 15 after the outer peripheral surface of the work W has been machined, the first tool rest 13 is moved back to a standby position B sufficiently away from the area for machining the work W as shown in FIG. 9(b), and then the second tool rest 15 is moved from the standby position D to a machining start position E for starting the machining of the work W, as shown in FIG. 9(c).
On the other hand, recently, because of a desire for a further reduction in work machining costs, measures have been taken to reduce machining time by, for example, reducing the time for indexing the tools by the tool rest and increasing the rotation speed of the spindle. However, the limit has been substantially reached in recent years in the reduction of the machining time achieved by the reduction of the index time and the increase of the rotation speed of the spindle, such that a further considerable reduction in the machining time can hardly be expected.
Therefore, the applicant of the present application has proposed a method of machining a work which reduces the time for switching the tools when machining the work while switching the tools of a plurality of tool rests, such that the machining time can be reduced and the machining costs of the work can be further reduced (refer to Patent documents 1 and 2).
Patent document 1: Japanese Patent Publication Laid-open No. 2002-341913
Patent document 2: Japanese Patent Publication Laid-open No. 2002-341915